WASHINGTON -- NATO's top commander said the massive influx of refugees into Europe is providing cover for terrorists and criminals and that poses increasing danger for an attack.

U.S. Air Force General Philip Breedlove also told a Senate committee on March 1 that Russia's military campaign in Syria -- now in its six month -- has "wildly exacerbated the problem" of refugees fleeing the civil war there.

He accused Moscow and the regime of Syrian President Bashar al-Assad of using migration as a weapon to overwhelm European support structures and break European unity.

"This criminality, the terrorists, and the returning foreign fighters are clearly a daily part of the refugee flow in Europe," he told the Senate Armed Services Committee.

He said Syria's use of barrel bombs have no military purpose and are aimed at terrorizing Syrian citizens to make them refugees and turn them into problems for other countries.